This section provides background information related to the present disclosure which is not necessarily prior art.
As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                  ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ×    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline material (1.455 g/cc).
Unfortunately, PET is a poor barrier to oxygen. One of the main factors that limit the shelf life of foods and beverages (herein known as “fills”) in PET containers is the ingress of oxygen through the walls of the container followed by oxidation of the fill. Many strategies have been employed to reduce the amount of oxygen in contact with food in PET containers. Some strategies include headspace replacement, which replaces oxygen in the headspace during packaging with an inert gas, such as N2 or C02. Alternative strategies include using package barrier coatings, such as chemical vapor deposited (CVD) aluminum or silicon. Still further, some strategies include the use of embedded barrier layers, such as multilayer packages, or PET barrier additives that create physical barriers to oxygen diffusion through the packaging (e.g., nylon, nanoclays). Finally, some strategies have used oxygen scavengers that react with oxygen in a predetermined way (e.g., oxidizable plastics, hydrogen gas, reactive metals & organic molecules) to minimize its effect, which usually requires the use of a catalyst.
An example of oxygen reducing technology is available from ColorMatrix (International Publication Number WO 2008/090354 A1, which is hereby incorporated by reference). The technology involves the slow release of hydrogen from the container using a hydrogen generator. The hydrogen subsequently reacts with oxygen in the presence of a metal catalyst to create water. Hydrogen that does not react with oxygen will slowly permeate out of the container.
However, the ColorMatrix system is predicated on the chemical reaction between the hydrogen generator, such as sodium borohydride, and a catalyst. Due to the need for water, in some embodiments, for the generation of the molecular hydrogen, placement of the sodium borohydride and water in the container can be critical. However, in some application, placement of the sodium borohydride, such as in the closure or closure shell of the container, may limit the amount of water (i.e. moisture) reacting with the compound (i.e. sodium borohydride) because other components used in the container, namely HDPE and PP, may be hydrophobic and, thus, limit the permeability of water.
Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching an injection molded PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque, and thus, generally undesirable. Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity for those portions of the container having biaxial molecular orientation. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of approximately 250° F.-350° F. (approximately 121° C.-177° C.), and holding the blown container against the heated mold for approximately two (2) to five (5) seconds. Manufacturers of PET juice bottles, which must be hot-filled at approximately 185° F. (85° C.), currently use heat setting to produce PET bottles having an overall crystallinity in the range of approximately 25%-35%.